If else statement in JavaScript keeps returning last else statement no matter user input

 var age = prompt('How old are you?');
    if ('age' <= 5){
        alert("You are a baby!");
    }
    else if ('age' >= 5 && 'age' < 18){
        alert("You are a child!");
    }
    else if ('age' >= 18 && 'age' < 70){
        alert("You are an adult!");
    }
    else
        alert("You are a senior!");

So I am trying to make it change depending on user input for age. However, no matter the age I enter “You are a senior!” is the only alert that goes off.